Muffler companies usually use pure metal sheet with thickness less than 1 mm, and drill holes with a diameter less than 1 mm on the sheet, and the drilling rate is 1% - 3%. By selecting mufflers with different perforation depth and plate thickness, the frequency spectrum characteristics of mufflers can be effectively controlled, and the silencing effect can reach the required frequency range. The structure of the hole muffler is that the end is closed and the pipe wall is straight, and many small holes are drilled in the pipe wall. The principle is based on the spectrum characteristics of the jet noise. The automobile muffler uses many small nozzles to replace it while keeping the total area of the nozzle unchanged. When the airflow passes through the small hole, the spectrum of the jet noise will move to high frequency or ultra-high frequency, which will significantly reduce the audible components in the spectrum, thus reducing the interference and damage to human body. The basic principle of active silencing device is to use electronic equipment to create sound waves with the same size and opposite phase as the original sound field in the original sound field, so that it can counteract the original sound field within a certain range. The utility model is an instrument and equipment, which is mainly composed of microphone, amplifier, phase-shifting device, power amplifier, loudspeaker, etc.